1. Field of Use
The invention subject of this application pertains to a method and apparatus for measuring the subterranean lithology using electromagnetic energy. The invention is applicable to the measurement from within uncased boreholes, i.e., and open-holes. The present invention transmits and receives electromagnetic energy deep into a geologic formation from a borehole, while also providing high resolution for distant measurements and the ability to control the direction in which the measurements are taken. The invention does not require a receiver to be located at the ground surface or in another borehole. In simple terms, the apparatus of this invention is able to “look around” into the surrounding geologic formation from a stationary position.
2. Description of Related Art
There has long been a need for an open-hole logging tool that would be capable of providing measurements of the lithology of a geologic formation in selected directions, providing measurements of the lithology both close to the bore hole and deep into the formation, and provide all such measurements with high vertical and lateral resolution. Existing logging tools can not provide the adequate penetration into the geologic formation surrounding the borehole. In addition, existing logging tools are not directional. The resolution of measurements is also limited, particularly at greater distances into the geologic formation.
The depth into geologic formations that existing tools, utilizing electromagnetic energy and located in a single borehole, can measure is limited by the separation between the transmitter and receiver for the electromagnetic energy. In general terms, this requires that to achieve an eight-foot depth penetration into the formation, the transmitter and receiver must be separated by at least eight feet. Further, a fixed separation distance limits the vertical resolution (assuming the axis of the borehole is vertically oriented) as measurements are attempted further into the formation from the borehole). As the depth of penetration of the electromagnetic energy is increased, the resolution of the measurement rapidly diminishes. To compensate for this loss of resolution, the transmitter and receiver separation must be increased.
Current methods depend upon various types of electromagnetic energy, either electric waves or magnetic waves, in the electromagnetic spectrum. The electric waves, more commonly known as radio waves, have an advantage in being able to be used at very high frequencies. At these high frequencies of megahertz to gigahertz, temporally pulsed waves may be used to determine the distance of an object. A well-known example of this technology is radar. However, these electric waves suffer great attenuation when confronted by ground water, clays or other highly conductive media within a geologic formation. Using focusing antennas with high gain improves the situation only marginally since the amount of gain is usually not enough to offset the amount of loss or attenuation of the electromagnetic energy incurred as the high frequency wave passes through layers of electrically conductive material.
Oscillating magnetic flux has the potential to achieve deeper penetration through geologic formations containing electrically conductive material than electric waves. There has not, however, been a high gain magnetic antennas available to focus the magnetic flux in a desired or controlled manner. Accordingly, even if the oscillating flux can penetrate a further distance into the ground, the signal rapidly dissipates. The signal decreases as the inverse cube of the distance of the intended target from the flux-generating source. The resulting rapid loss of power has substantially limited the effective range of distance that oscillating magnetic flux can be utilized.